The invention relates to a system for the electronic editing of video-signals recorded by means of a diagonal-track-method on magnetic tapes for a video-recording/play-back-unit.
A typical prior art recording/playback-unit is known from the periodical "Radio Mentor Electronic", volume 41 (1975), #10, pages 402 to 403. In order to obtain as accurate an editing as possible for a diagonal-track-method, it is proposed according to the German Patent DT-OS No. 14 62 433 to imprint new recordings over the non-erased video-tracks. This presents a problem because the first recording is not erased, but is only attenuated. Thus, disturbing interferences can arise in the transition region during the play-back of the new recording. It is known from the periodical "Fernseh-, und Kino-Technik: (Television-, and Movie-Techniques)", 1971, No. 8, pages 279 to 285, that this disadvantage can be eliminated by using erasing-heads which rotate and are associated with respective video-heads of the rotating headwheel, so that the erasing-heads erase each diagonal-track shortly before the new recording.
Generally, two different editing types are distinguished:
In the case of the first editing type, the individual scenes are joined together, so that the new recording is joined free of disturbances to a present recording, so that during play-back, no discontinuities in the synchronous signal can occur to make the instrument lose synchronism or bring about image-disturbances. Customarily, a stationary main erasing-head which serves for the simultaneous erasing of video-, sound-, and control tracks and has to be arranged ahead of the head-wheel, is switched-in at once for this purpose. The rotating video-, and erasing-heads are switched-in and switched-out of the circuit by means of a control device in accordance with a time sequence dictated by the construction of the instrument.
In the case of a second editing type, individual video-tracks are newly inserted or imprinted in a present recording. Different tracks, in particular the control-track, are not erased by the main erasing-head in this editing type. As a result of this, the capstan-motor can be readjusted by means of the reference-signal which can be removed from the control-track, so that a disturbance-free edit-start and edit-end can be achieved. In the case of this editing type, the control-unit undertakes the time-wise accurate coordination of the heads present on the rotating head-wheel with respect to the momentary location of the moving magnetic tape.
In the case of prior art editing systems, the control unit includes individual delay-stages or timing-elements, for example, of monostable flip flops, which are temperature dependent and voltage dependent and thus can cause disturbances in the course of the switching operations.
Typically, these timing-elements are switched in series, whereby each timing-element is controlled by the preceding timing-element so that cumulative errors can occur.
It is the goal of the present invention to devise a system of the initially mentioned type, which is independent of the individual and, in part differently constructed timing-elements, which in turn control the accurate editing-course of the change-over points of time of the heads located on the head wheel.